The plastic injection molding process typically utilizes two phases or stages—a plasticizing phase or stage followed by an injection phase or stage. Plastic injection molding machines usually utilize a heated barrel in order to plasticize or transform pelletized or granular thermoplastic material into the molten state. The barrel has a reciprocating auger-type feed screw mounted therein which forces the molten thermoplastic material through the outlet end of the barrel for injection into a die. During the plasticizing phase or stage, the feed screw rotates and forces the pelletized or granular thermoplastic material toward the forward end of the screw and barrel. As the pelletized or granular thermoplastic material flows toward the forward end of the feed screw and barrel, it is transformed into the molten state by a combination of frictional heat generated by the movement of the thermoplastic material against the screw and the inner surface of the barrel and conductive heat transferred through the wall of the barrel from electrical resistance heaters mounted on the exterior of the barrel. Continued rotation of the feed screw results in the molten thermoplastic material flowing to the end of the screw where it passes through a check valve into a cavity. As the molten thermoplastic material is received within the cavity, a differential pressure develops across the ends of the feed screw causing the feed screw to move toward the feed end of the barrel while the screw rotates. After a predetermined volume of “shot” of molten thermoplastic material is received within the cavity in front of the check valve, rotation of the feed screw is stopped by associated controls. The injection phase or stage then commences causing the feed screw to move forward toward the end of the barrel whereupon the check valve closes in response to flow of molten thermoplastic material. The “shot” of molten thermoplastic material now remaining in the cavity end of the barrel is then forced through a nozzle and into the die.
There are a number of commonly used check valves that are responsive to the backflow of molten thermoplastic material in order to close same. These valve types include floating rings, reciprocating balls, and poppets. The poppet styles vary in configuration with some poppets utilizing complicated and failure-prone spring arrangements. Although the valves presently used in the injection molding industry meet with varying degrees of success, there still remains a need for a reliable, fast acting poppet valve in some applications. This is especially true where precise shot control is critical.
In view of the foregoing limitations associated with presently available check valves for use in plastic injection molding machines, it has become desirable to develop a check valve that utilizes a uniquely responsive poppet member that does not require spring loading and wherein the poppet member is configured so that it is significantly more responsive to the backflow of molten thermoplastic material, thus resulting in the rapid closing of the check valve virtually immediately after the commencement of the injection phase or stage.